1. Field of the Invention
This invention is related to zero-voltage, resonant transition switching DC-DC interleaved bridge converters
2. Description of the Background Art
Modern DC-DC power converters generally include semiconductor switching elements, which when turned on and off couple energy from a DC source to an inductor in the converter. This energy is then transferred from the inductor to the DC output. Common DC-to-DC converter topologies include the buck (or forward) converter, the buck-boost (or flyback) converter, and the boost converter topologies.
Reliability, low cost and efficiency are all important considerations in power supply design. However, the features of high efficiency and high reliability generally are at odds with the feature of low cost. High reliability power supplies often contain bulky tantalum capacitors, a number of isolation transformers and smoothing chokes, and a variety of snubber components.
It is known that zero-voltage, resonant transition (ZVRT) switching can substantially reduce switching losses. ZVRT is the technique of changing the state of a control switch during a short duration of time when the voltage across the switch is approximately zero to substantially eliminate switching power losses in the switch. Examples of an interleaved flyback converter that uses such techniques is shown in U.S. Pat. No. 4,618,919, issued Oct. 21, 1986, entitled "Topology for Miniature Power Supply with Low voltage and Low Ripple Requirements" to Hubert C. Martin, Jr. and assigned to the assignee of the present invention. A full-bridge ZVRT multi-resonant converter was published in the Applied Power Electronic Conference Proceedings, pages 109-118 in March 1990 by Milan M Jovanovic, Ching-Shan Leu, and Fred C. Lee under the title "A Full-Bridge Zero-Voltage-Switched Multi-Resonant Converter for Pulse-Load Applications."
A number of U.S. Pat. Nos., including 4,823,249, Garcia, II; 4,747,035, Bobry; 4,739,461, Komatsu et al; 4,706,181, Mercer; and 3,659,185, Gregorich, show that power supply and converter circuits can be implemented with a single transformer. An article entitled "A New Non-Dissipative Load-Line Shaping Technique Eliminates Switching Stress in Bridge Converters" by Ron Goldfarb, which appeared in Proceedings of Powercon 8, D-4 pages 1-6, shows a non-dissipative bridge converter of a type that is described below with reference to FIG. 1. A full-bridge power converter with zero voltage resonant transition switching is shown in U.S. Pat. No. 4,953,068, issued Aug. 28, 1990, entitled "Full Bridge Power Converter with Multiple Zero Voltage Resonant Transition Switching" to Christopher P. Henze and assigned to the assignee of the present invention.
An interleaved forward DC-DC power converter with interleaved timing is disclosed in copending U.S. Pat. application Ser. No. 498,863, filed Mar. 23, 1990 in the names of David W. Parsley and Hubert C. Martin, Jr., which is assigned to the assignee of the present invention. In this invention two buck switching power stages are operated 180.degree. out-of-phase, with the input nodes of a pair of inductors being coupled to a common filter capacitor. A secondary coil is wound on each inductor core which is switched to a load at the time the primary winding is shunted across the input filter capacitor. In this way, dual-inductor buck power operation is provided while maintaining input-output isolation.
An improved converter of the buck switching type is described in U.S. Pat. No. 4,713,742, "Dual-Inductor Buck Switching Converter," invented by David W. Parsley and assigned to the assignee of the present invention. In that invention a DC-to-DC power converter provided power to two parallel-connected inductors in series with a DC input power source. Commutation switches connected in series with each of the inductors were controlled to have a duty cycle that is greater than 50%. This duty cycle provided interleaved timing such that whenever one of the switches turns on or off, the other switch is still closed during the switching time. As a result of this construction, the sum of the currents that flow through the inductors provides an output current to the load with greatly reduced ripple and substantially constant output current.
In U.S. Pat. No. 4,618,919, a dual transformer switching topology provided isolation of the input and output power terminals. A storage capacitor applied at the input of the power supply was switched sequentially in accordance with an interleaved timing to provide continuous filter support for the output voltage produced by the power supply. The capacitor performed a filtering function for the output voltages and also stored energy at high voltages as a result of the turns ratio of the power supply transformer. This permitted the use of a capacitor with reduced physical size and capacity. The Martin patent was of the interleaved flyback converter type which, however, lacks inherent stability over a wide band of attenuation of input ripple.
In a paper entitled "Zero-Voltage Switching in High Frequency Power Converters Using Pulse Width Modulation," by C.P. Henze, et al, IEEE Applied Power Electronics Conference (APEC) Record, pp. 33-40, February 1988, a zero-voltage switching technique employing resonant transition during a switching interval of short, but finite, duration was discussed. This technique is applicable to conventional power converter topologies and offers efficient operation at very high switching frequencies while retaining the fundamental characteristics of the conventional topology. With this design power transistor parasitic capacitor switching losses are eliminated, and the voltage stress of the power transistor during changing state was not increased, but these advantages were obtained at the expense of increased conduction losses and a requirement for synchronous rectification.
In U.S. Pat. application Ser. No. 492,708, filed Mar. 12, 1990 in the names of Christopher P. Henze, David S. Lo and Hubert C. Martin, Jr. and assigned to the assignee of the present invention, a single ended DC-DC power converter is disclosed which may operate with zero-voltage resonant transition switching at very high switching frequencies (1 MHz and greater). Only one magnetic element is required, which acts as both an inductor and a transformer. The turns ratio may be varied to obtain a desired voltage conversion ratio. Control-to-output characteristics are identical to conventional power converter topologies. The circuit inherently allows for zero-voltage, resonant transition switching and magnetic isolation without making special demands on the magnetic element.